1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copying machine, a facsimile, etc., employing electrophotography, ink jet recording, etc.
2. Description of the Related Art
Various image forming apparatuses, such as copying machines, facsimiles and the like, form images by electrophotography, ink jet recording, etc., while conveying recording members, such as sheets of paper.
A conventional electrophotographic copying machine will be described with reference to FIG. 16.
This image forming apparatus is essentially composed of an image input section 90R and an image output section 90P. The image input section 90R inputs image information to be recorded, processes the information in various manners, and then transmits the image signals generated from the information to the image output section 90P.
The image output section 90P is essentially composed of: an image forming unit (photosensitive drum) 91 for forming an image to be recorded; a sheet feeder unit 92 for housing sheets S and feeding a sheet S one at a time; a transfer/conveyance unit 93 for receiving a sheet S from the sheet feeder unit 92, and conveying the sheet S to the image forming unit 91 while retaining the sheet S on the surface of a transfer drum, and transferring the image from the image forming unit 91 onto the sheet S; a fuser unit for fixing the image recorded on the sheet S; and a control unit 95 for controlling the operation of the entire apparatus.
The procedure of recording operation of the image forming apparatus will be described in detail.
When the recording operation is initiated, the sheet feeder unit 92 feeds one sheet S at a time. After the sheet is conveyed to a pair of register rollers 96 provided immediately upstream from the transfer/conveyance unit 93, the register rollers 96 start to rotate.
The register rollers 96 start to rotate synchronously with an image recording initiation signal, thus conveying the sheet S to the transfer/conveyance unit 93. In the transfer/conveyance unit 93, when the sheet S is further conveyed through a guide portion 93A to a transfer drum 93B, the sheet S is attached to the surface of the transfer drum 93B. While the transfer drum 93B rotates, the sheet S is conveyed to an image transfer region T, where the image formed by toner on the image forming portion is transferred onto the sheet S.
Conventional image forming apparatuses employ various methods to retain a sheet S on the surface of a transfer drum, for example: an electrostatic method in which a sheet S is electrostatically attached to a conveying medium formed of a thin dielectric film, a suction method in which a sheet S is drawn to the surface of the drum by air suction, or a combined method in which edges of a sheet S are clamped by a gripper while the sheet is electrostatically attached or drawn by air suction to the drum.
After recording has been thus performed on the sheet S, the sheet S is conveyed, as the transfer drum 93B rotates, to the vicinity of a fuser unit 94. Then, the sheet S is peeled off from the transfer drum 93. The peeling is started from the leading edge of the sheet S. While the sheet is being peeled from the transfer drum 93, the sheet S is continuously conveyed toward the fuser unit 94. The fuser unit 94 applies heat or pressure to the sheet S, thereby fixing the toner image thereon. Finally, the sheet S is ejected by a pair of ejection rollers 97 onto a tray 98 provided on the outside wall of the apparatus.
If a conveyance failure (sheet jam) occurs during the recording operation, the image forming apparatus stops the operation and indicates the conveyance failure on a display (not shown) to instruct the user to perform jam recovery, for example, by removal of the stuck sheet S.
Therefore, one of major requirements for an image forming apparatus is that the apparatus is constructed so as to facilitate jam recovery, thereby substantially eliminating the possibility that during jam recovery the user tear the stuck sheet S and leave a fragment of the sheet S in the apparatus or mistakingly damage a component part of the apparatus.
During the recording operation to the above-described image forming apparatus, conveyance failure is most likely to occur in the transfer/conveyance unit 93, because the unit 93 has a complex structure. To facilitate safe jam recovery without a failure, the transfer/conveyance unit (a portion indicated by a dotted-line box) 93 has a construction that allows the unit 93 to be drawn out from the image forming apparatus 99. A sheet S can be easily removed from the transfer drum 93B after the transfer/conveyance unit 93 has been drawn out.
When the transfer/conveyance unit 93 is drawn out, a user can more easily see a jammed sheet S and perform the sheet jam recovery than in a construction where the unit 93 cannot be drawn out and in which a user must put his/her hand into the image forming apparatus 99 to perform the sheet jam recovery. Further, the above-described construction will substantially eliminate a danger that while performing the sheet jam recovery, a user may mistakingly deform or break the conveyance medium, such as a thin film, that is retaining the sheet S.
However, even in the conventional construction that allows the transfer/conveyance unit 93 to be drawn out, it is not easy to remove a jammed sheet S if the sheet S is held by both the sheet feeder unit 92 and the transfer/conveyance unit 93.
For example, if a leading edge portion of a sheet S fails to completely attach to the attachment film of the transfer drum 93B when the sheet S is being conveyed from the register rollers 96, the leading portion of the sheet S may go into the image forming unit 91. When such a malfunction is detected, the conveyance of the sheet S must be stopped immediately in order to prevent the sheet S from damaging the image forming unit 91. When the conveyance of the sheet S is thus stopped, a rear portion of the sheet S is, often, still clamped between the register rollers 96, thus impeding sheet jam recovery as follows. If the transfer/conveyance unit 93 is drawn out of the image forming apparatus, the sheet S held by both the attachment film and the register rollers 96 will be torn, or the attachment film will be deformed or broken.
To avoid such problems, the register rollers 96 are manually rotated in the reverse direction to move back the sheet S before the sheet jam recovery is performed. However, when the register rollers 96 are rotated backward, the attachment film is pulled back along with the sheet S and receives a considerably large load because the transfer drum 93B is in contact with attaching means and cleaning means (not shown) and thereby opposes a large rotational load.
The image forming apparatus can be stopped with a sheet S held by both the register rollers 96 and the transfer/conveyance unit 93, not only when a conveyance failure is caused by the sheet S in the transfer/conveyance unit 93 but also, for example, when a conveyance failure is caused by the preceding sheet S downstream from the transfer/conveyance unit 93. Although the incidence of such a stoppage of the image forming apparatus is quite high, the conventional image forming apparatus having the above-described construction that allows the transfer/conveyance unit 93 to be drawn out lacks means for facilitating sheet jam recovery in such a stoppage. On the other hand, it is impractical to provide sheet jam recovery mechanisms respectively adapted to the individual types of sheet jams. It would increase the size and costs of the apparatus and complicate sheet jam recovery operations.